This invention relates a method of controlling a vehicle transmission. ‘Vehicle transmission’ here refers to a transmission to be interposed between a prime mover of the vehicle and ground engaging means of the vehicle which provides a plurality of gear ratios by which the ground-engaging means can be driven by the prime mover. The transmission has been devised primarily, but not exclusively, for a material handling vehicle, such as, for example, a tractor, loader type or similar vehicle for industrial and/or agricultural use, in which the ground engaging means typically comprises wheels but could also comprise crawler tracks.
Vehicles such as tractors or loaders have conventionally used transmissions which provide a range of forward gear ratios. It is a requirement of such transmissions that the vehicle needs to develop useful tractive effort over a wide range of speeds. For example, in bottom gear the vehicle may be designed to have a maximum speed of 1.5 kph and in top gear have a maximum speed of 65 kph, to permit the vehicle to operate at low speed whilst under load, but enable the vehicle to travel at an adequate speed when travelling along a road.
To provide a wide range of close ratios, it is known to provide a transmission having effectively a pair of gear boxes, a first gear box drivingly connected to the engine and providing a first range of ratios and a reverse gear, and a second gear box connected to the output of the first gear box and providing a plurality of selectable gear ranges. The output of the second gear box is then connected to the ground engaging means. Consequently, where the first gear box provides six gears and the second gear box provides four gear ranges, it will be apparent that there are 24 torque paths through the transmission. Advantageously, gear selection is performed by an electronic control unit which supplies fluid to power shift clutches or operates a solenoid to move synchromesh clutches in response to a gear shift demand from the operator.
To engage and disengage the transmission from driving connection to the prime mover, the transmission is further provided with a main clutch which is connected between an input from the prime mover, directly or indirectly, and an input shaft of the transmission. As the master clutch has to transmit a high torque, conventionally such clutches use large steel counter-plates and are filled with oil.
A disadvantage with such a main clutch is that, even if the clutch is disconnected, there will be some viscous drag due to the oil in the clutch which will transmit some torque through to the gearbox. If the gearbox is not in neutral, this can lead to the vehicle creeping forward and so will require the operator to hold the vehicle with the brake. It will be apparent that the clutches in the gearbox will be disconnected, but the transmitted torque from the master clutch will still drive the input gearing of the gearbox. Consequently, when the power shift clutches of the gearbox are engaged to select a gear and move off, the energy in the input gearing arising from the drag torque in the master clutch will cause a noticeably jerky start of the vehicle.